This invention relates to tractor-drawn scrapers having a pivotal apron mounted on the scraper bowl and more particularly to a lift linkage assembly for opening and closing the apron.
U.S. Pat. Nos. 3,553,860 and 3,854,381 illustrate the type of tractor-drawn scrapers to which the present invention is directed and show the presently conventional form of lift linkage assembly used for raising and lowering the apron. In particular, such lift linkage assembly has one end of the linkage mounted on the rear of the tractor unit, and the other end connected to the apron on the bowl unit and a hydraulic jack connected between the linkage and tractor unit for actuating the linkage to cause the apron to move between open and closed positions.
Such tractor-down scrapers also include lift jacks connecting between the rear of the tractor unit and the forward end of the bowl so that the forward end of the bowl can be raised or lowered relative to the tractor unit and to the ground in order to vary the elevation above ground of the cutting edge on the scraper bowl.
The degree of tangential closing force applied to the apron is of importance since in closing the apron from open position the lower edge to the apron must oftentimes be forced through the material being loaded into the scraper. Considerable force must be exerted at times to shear through material that is obstructing closing movement of the apron and blocking it open.
It has been found that with present conventional apron life linkage assemblies the apron closing force will vary in accordance with the elevation of the front end of the scraper bowl above ground. Also, the connection of the linkage assembly between the rear end of the tractor unit and the bowl apron closing force is needed to shear through an obstruction to closure.
Additionally, it is common practice to raise the front end of the bowl while simultaneously moving the apron downwardly to closed position, requiring use of sequence valves, of the type shown in the aforesaid U.S. Pat. No. 3,854,381, to coordinate the operation of the bowl lift and apron lift hydraulic jacks.
It is the primary object of the present invention to provide an apron lift linkage assembly which is a greater degree of tangential closing force over a considerable degree of movement to closed position, in which the closing force does not vary with changes in elevation of the bowl, which eliminates the need for a sequence valve, and which does not exert a strain on the tractor unit when a high shearing force is applied to the apron.